For a long time, estimated to be between 10 and 20 years, the Internet will have a mixture of IPv4 and IPv6 which will need to communicate with each other. Many of these communications needs can be facilitated through middleboxes of one sort or another. It is difficult to give general advice about which middleboxes to install in your network, but you should survey the possibilities and see whether or not one of them will work for you.

Application Proxies

This is just a variation on the ALG Application Layer Gateway but instead of addressing the need for security, it bridges the gap between IPv4 and IPv6. It can be as simple as an SMTP relay server that functions on both IPv4 and IPv6 simultaneously.

NAT-PT

This is a form of NAT with additional feature that are needed to bridge the gap between IPv4 and IPv6 hosts. It is defined in RFC 2766. Although the IETF doesn't particularly like this solution, it has been implemented by some vendors such as Cisco, and it will be helpful to many ISPs during the transition to a pure IPv6 Internet.

Web Proxies with CONNECT Support

This might be a general solution for you if you have a userbase which will be satisfied with operating behind a web proxy. There are more details in this Internet Draft written by Iljitsch van Beijnum.